Ventilation systems and air conditioning systems commonly use a fan comprising a fan blade or impeller driven by an electric motor to create the air flow and adjust the air flow by changing the speed of the motor. When the static pressure of the system varies, which may be caused by changes in the ductwork of the system, in exterior wind pressure or the like, the volume of air moved by the system also varies. Such as the static pressure in the system increases because dust is trapped in an internal device of the system, in particular, oil stain is attached to a filter screen of a range hood after operation for a long time, the air volume is decreased, thereby resulting in polluting indoor air. The increasing of carbon monoxide which will endanger the health of users in kitchens. Therefore, for certain installations it is desired to keep the volume of air flowing through the system constant, regardless of static pressure variations.
The present invention aims to provide a new method to keep the volume of the air flow constant.